Electrical bushing having a capacitor chain formed by overlapping capacitor elements

ABSTRACT

An electrical bushing having electrical insulation disposed about an axially extending conductor, and a plurality of stress grading elements disposed in the insulation to grade radial and longitudinal stresses in the bushing. These stress grading elements includes at least two groups of substantially tubular elements, with the elements of each group being axially spaced and dimensioned such that the wall portions of each group are substantially aligned. The aligned space between two elements of one group is bridged by an element of an adjacent group.

United States Patent Friedrich [151 3,692,928 [4 1 Sept. 19, 1972 [54]ELECTRICAL BUSHING HAVING A 3,248,599 4/1966 Masuda ..174/ 143 X FOREIGNPATENTS OR APPLICATIONS 656,127 l/l963 Canada ..174/73 R 289,850 8/1928Great Britain 174/143 Primary Examiner-Laramie E. Askin AttorneyA. T.Stratton and Donald R. Lackey [5 7] ABSTRACT An electrical bushinghaving electrical insulation disposed about an'axially extendingconductor, and a plurality of stress grading elements disposed in theinsulation to grade radial and longitudinal stresses in the bushing.These stress grading elements includes at least two groups ofsubstantially tubular elements, with the elements of each group beingaxially spaced and dimensioned such that the wall portions of each groupare substantially aligned. The aligned space between two elements of onegroup is bridged by an element of an adjacent group.

12 Claims, 4 Drawing Figures PATENTEIISEP 19 I972 WITNESSES g a/ J7 vFIG.2.

ATTORNEY ELECTRICAL BUSHING HAVING A CAPACITOR CHAIN FORMED BYOVERLAPPING CAPACITOR ELEMENTS BACKGROUND OF THE INVENTION 1. Field ofthe Invention The invention relates in general to electrical bushings,and more specifically to high voltage electrical bushings suitable foruse with power transformers and power circuit breakers.

2. Description of the Prior Art High voltage electrical bushings forpower transformers and power circuit breakers include a plurality ofcapacitor plates or elements disposed in the insulation surrounding theaxially extending electrical conductor, to more uniformly distributeelectrical stress throughout the insulation, and thus utilize theinsulation more effectively. The proper positioning of the capacitorelements, and maintaining the elements in position, was not a problemwith the prior art condenser type bushings which built up the capacitorsection by winding paper about a form, as metallic foils of the desiredlength and width could be easily placed between the turns of the paperat the required intervals as the capacitor section was being wound.However, with the development of high strength, weather resistant,castable, non-tracking solid resinous insulation systems, it becamedesirable to form the capacitor section of certain types and ratings ofelectrical bushings, with the capacitor plates being disposed throughthe resulting cast solid insulating structure. This replaces the oilimpregnated paper capacitor assemblies of the prior art, as well as thefragile, porcelain shells used to contain the oil and provideweatherproof housings for the capacitor section. The cast condenserbushing, however, introduces manufacturing problems in an entirely newarea, as the thin metallic or semiconductive capacitor foils or platesmust be accurately positioned within the casting mold, and theirpositions must be maintained while a highly filled liquid casting resinsystem is introduced into the mold, and subsequently gelled and cured toa solid. While many workable solutions to the problem have beenproposed, they have approached the problem by using the same generaltype of capacitor arrangement used with the prior art paper typecondenser bushings. It would be desirable to simplify the capacitorstructure of the cast type bushings, if this could be accomplishedwithout incurring offsetting disadvantages.

SUMMARY OF THE INVENTION Briefly, the present invention is a new andimproved high voltage electrical bushing of the cast condenser type.However, instead of employing a large plurality of radially spacedcapacitor foils, as used in prior art wound type condenser bushings, anew and improved capacitor structure is utilized which lends itself tocast type bushing construction.

More specifically, it was found that with solid, cast resin insulationsystems, a more effective capacitor structure would be one in which theprimary function of the capacitor or stress grading elements is to gradethe electrical stresses longitudinally, with the radial grading of theelectrical stresses being of lesser importance. The cast solid resinousinsulation systems are excellent in puncture strength, and thus they donot require the same radial grading that a capacitor type bushingrequires which is formed of paper impregnated with oil. However, thelongitudinal grading must be highly efficient in order to uniformlydistribute the voltage stresses across the length of the bushings, andthus reduce the magnitude of local stress concentrations which maypromote creepage along outer surfaces of the bushing.

The capacitor structure is formed by utilizing at least two groups ofsubstantially tubular stress grading elements, disposed coaxially aboutthe bushing conductor. The elements of each group are axially spacedfrom one another and dimensioned such that their wall portions aresubstantially aligned. The two aligned groups are radially spaced, andaxially offset to bridge each space between two elements of one groupwith an element from another group, with the bridging elementoverlapping the ends of the two spaced elements in the adjacent group.Thus, a capacitor chain is formed from each end of the electricalbushing to the ground layer of the bushing, which uniformly distributeslongitudinal electrical stresses, and the radially spaced groups providethe required ra'dial grading of stress. The reduced number of radiallayers substantially reduces the complexity of casting the capacitorsection, as adding axially spaced elements to cast type bushings doesnot present the manufacturing problem that a relatively large pluralityof radial elements presents.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be betterunderstood, and further advantages and uses thereof more readilyapparent, when considered in view of the following detailed descriptionof exemplary embodiments, taken with the accompanying drawings, inwhich:

FIG. 1 is an elevational view, partially in section, of an electricalbushing constructed according to a first embodiment of the invention;

FIG. 2 is a perspective view of the conductor and stress grading elementarrangement for the electrical bushing shown in FIG. 1, along with aschematic representation of the resulting capacitive structure;

FIG. 3 is a fragmentary elevational view, partially in section, of anelectrical bushing constructed according to another embodiment of theinvention; and

FIG. 4 is a fragmentary, elevational view, partially in section, of anelectrical bushing constructed according to still another embodiment ofthe invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, andFIG. 1 in particular, there is shown an elevational view, partially insection, of a high voltage electrical bushing 10 constructed accordingto a first embodiment of the invention. Bushing 10, which may be usedwith power transformers or power circuit breakers, has a generallyelongated shape, and includes a centrally or axially extendingelectrical conductor 12 having first and second ends 14 and 16,respectively, formed of a good electrical conductor, such as copper oraluminum. Conductor 12 may be threaded adjacent to its ends 14 and 16,as illus trated at 18 and 20, respectively, in order to provide meansfor connecting external and encased electrical leads thereto,respectively.

Electrical conductor 12 has an insulating body member or portion 22disposed thereon having first and second ends 24 and 26, respectively,which ends are spaced from the first and second ends 18 and 20,respectively, of the electrical conductor 14. Insulating body member 22is formed of a cast solid resinous insulation system. The cast resinousinsulation system is preferably thermosetting, but thermoplastic resinsystems may be used if their softening temperatures are well above themaximum operating temperature of the electrical bushing, and they arecompatible chemically with the liquid dielectric disposed in theassociated apparatus. In general, the resin system of which the bodyportion 22 is formed should be mechanically strong, weather resistant,it should have a low shrinkage factor upon curing, it should providegood adhesion to conductor l2, and have excellent crack resistance uponthermal cycling. The resinous polymeric epoxides have been found to beexcellent.

The resin system selected may be filled with suitable finely divided,inorganic fillers, to obtain nontracking characteristics, to reduceshrinkage upon curing of the resin system, to more closely match thecoefficient of thermal expansion of the resin system with the conductor12, and to provide other desirable characteristics. Suitable fillerswhich may be used are alumina trihydrate (Al o 31-1 quartz, and silica.

Mounting means 13 is disposed intermediate the ends 24 and 26 of thesolid insulation, and it includes a flange member 15 adapted to restagainst the casing 17 of the associated electrical apparatus, about anopening 19 disposed therein. The bushing is disposed through the opening19, and a gasket 21 may be disposed between the flange and the casing 17to seal the opening 19. The flange 15 may be an integral portion of thebody member 22, as illustrated in FIG. 1,

or it may be a separate member which is attached to the body member 22by any suitable means, such as an adhesive. The body member 22 includesa plurality of weather sheds 23 on the weather end of the bushing 12,and these weather sheds may be cast integrally with the remaining bodyportion of the bushing, or the cast body portion of the bushing may beformed with a smooth exterior on the weather end, and a separate weatherhousing disposed tightly about the smooth surface, which would theninclude the weather sheds.

The desired axial and radial stress grading of bushing 10 is provided bya new capacitor arrangement which utilizes a relatively large pluralityof axially spaced capacitor elements, enabling the number of radiallyspaced tiers of capacitor elements to be substantially reduced. Theconstruction of prior art cast type condenser bushings has largelyutilized the general capacitor element arrangement of the prior artwound type condenser sections. However, cast solid insulation systemshave a different electrical insulating characteristic than wound paperinsulating sections, and advantage of this fact is taken to change thestress grading approach, and as a result provide a capacitor structurewhich lends itself to the production of cast type high voltage bushings.

More specifically, FIG. 1 illustrates first and second groups of stressgrading elements or capacitor plates, indicated generally by referencenumerals 30 and 32, respectively. The first group 30 includes aplurality of substantially tubular stress grading elements 34, 36, 38,40, 42, 44, 46 and 48, disposed coaxially with the electrical conductor12. Each of the stress grading elements in the first group 30 has apredetermined longitudinal dimension, and the longitudinal dimensions ofthe elements may be the same for all elements of the group, or they maybe varied according to a predetermined plan. The longitudinal dimensionis illustrated with reference numeral 50 on stress grading element 46. v

The stress grading elements of the first group 30 are axially spacedfrom one another by a predetermined dimension, or dimensions, such asindicated by the reference numeral 52 between stress grading elements 46and 4 8. The axial spacing of the stress grading members of the firstgroup 30 may be uniform between adjacent pairs across the group, or thespacing may be changed in a predetermined manner. The stress gradingelements of the first group 30 may be formed of good electricalconductors, such as copper or aluminum, they may be formed of partiallyconductive material, such as carbon, or they may be formed ofsemiconductive material, i.e., material having a voltage dependentresistivity, such as paint containing finely divided silicon carbide.Further, it is not necessary that all of the capacitor elements of thefirst group be formed of the same types of material, as in certainapplications it may be desirable to utilize a combination of conductiveand semiconductive materials.-

In the embodiment of the invention shown in FIG. 1, the stressgradingelements of the first group 30 are in the form of straightcylinders, and it is important to note that the wall portions of thecylinders are substantially aligned from one end of the bushing 10 tothe other. The alignment of the wall portions greatly facilitates themanufacture of the bushing, as the stress grading elements may beapplied in the form of paint to an insulating tubular support memberwhich extends from one end of the bushing mold to the other, and thistubular support member may be formed of mesh, such as disclosed in US.Pat. No. 3,513,253, which is assigned to the same assignee as thepresent application, or this support tube may have a solid wall, asdesired. Alternatively, the stress grading elements of the first group30 may be formed by constructing the bushing with a plurality ofsuccessive casting operations, wherein a first body member is cast aboutthe electrical bushing conductor 12, with the diameter of the first bodymember being the required diameter for the stress grading elements.After the first body member is cast, the axially spaced stress gradingelements of the first group 30 may then be easily applied, such as bypainting the elements on the outer surface of the body member.Constructing a cast electrical bushing by successive casting steps isdisclosed in US. Pat. No. 3,394,455, which is assigned to the sameassignee as the present application.

The second group 32 of stress grading elements includes a plurality ofsubstantially tubular capacitor plates, such as plates or elements 56,58, 60, 62, 64, 66 and 68. The stress grading elements of the secondgroup 32 are axially spaced, and they have a larger diameter than thestress grading elements of the first group, to provide a predeterminedradial spacing between the first and second groups 30 and 32,respectively, such as the radial spacing 54 indicated between stressgrading elements 46 and 66. Each of the stress grading elements of thesecond group 32 has a predetermined longitudinal dimension, such as thedimension 70 indicated for element 64, and the longitudinal dimensionsof the elements of the group may be the same, or they may be varied in apredetermined pattern to provide specific capacitive values, as desired.The stress grading elements of the second group 32 are axially spaced,with the axial spacing between adjacent pairs being uniform, or varied,as desired. The axial spacing is indicated by reference numeral 72between elements 64 and 66 of the second group 32.

The second group of stress grading elements are axially offset, relativeto the first group 30, such that each space between adjacent pairs ofstress grading elements of the first group 30 is bridged by a stressgrading element of the second group. For example, the space 52 betweenelements 46 and 48 of the first group 30 is bridged by element 68 of thesecond group. Further, in addition to bridging the space, the elementsoverlap by predetermined dimensions to provide predetermined capacitivevalues between the overlapping elements of the two groups. An example ofthis overlapping of radially spaced elements is illustrated by referencenumeral 74, which illustrates the overlap between stress gradingelements 44 and 66.

If the bushing is constructed by using insulating support tubes disposedwithin the mold, thus requiring only one casting step, the second group32 of stress grading elements may be easily formed by providing a secondsupport tube of the required diameter, upon which the stress gradingelements are painted or otherwise suitably formed thereon.

If the first group 30 of stress grading elements was formed on a firstbody member cast about electrical conductor 12, a second casting stepcould be used to provide a second cast body portion, disposed about thefirst body member, which has a smooth outer surface upon which thestress grading elements of the second group 32 may be disposed. A thirdcasting step may then be used to complete the insulating body portion 22of the bushing, including the formation of the weather sheds 23, ifdesired.

One of the stress grading elements of the second group 32 may be used asthe ground layer of the bushing, or in the event that separate groundand tap layers are required, a separate ground layer 76 may be disposedadjacent to the mounting means 13 of the bushing 10. In this instance,the stress grading element 62 may be used as a tap layer, with a lead(not shown) being connected from the layer 62 to a terminal accessibleon the outer surface of the bushing 10. The ground layer 76 is connectedto the casing 17 during normal use of the electrical bushing, and thecasing 17 is grounded as shown at 78. The grounding of the layer 76 maybe accomplished by connecting the ground layer 76 via a suitableelectrical lead to a metallic sleeve or washer member disposed such thata mounting bolt, such as mounting bolt 23, may be telescopedtherethrough and make electrical contact therewith, when bolts are usedto mount the electrical bushing 10 to the casing 17, as illustrated inthe example of FIG. 1.

FIG. 2 is a perspective view of the conductor 12 and the first andsecond groups 30 and 32 of stress grading elements, with the conductorand stress grading elements being shown in phantom, in order toillustrate more clearly the resulting capacitor structure which isformed by the disclosed stress grading arrangement. As illustrated, eachof the stress grading elements of the first group 30 provides acapacitance between the element and conductor 12, such as capacitors 82and 86 shown connected between conductor 12 and stress grading elements34 and 36, respectively. Further, each of the overlapping stress gradingelements of the second group provides two capacitors between itself andthe two stress grading elements of the first group which it overlaps.For example, a first capacitor 84 is provided between one end of stressgrading element 56 and one end of stress grading element 34. A secondcapacitor 88 is provided between stress grading element 56 and stressgrading element 36. It will be noted that the disclosed arrangement ofstress grading elements provides a chain or ladder of capacitors whichextend from each end of the bushing to ground 78. The stress gradingelements at each end of the ladder, i.e., elements 34 and 48, arepreferably directly connected to conductor 12. Element 62 is connectedto ground 78 via the capacitance 90 between itself and the ground layer76. This arrangement of parallel and serially connected capacitorsuniformly grades the distribution of electrical stress from the ends 14and 16 of conductor 12 to ground 78, and the two radially disposedgroups of stress grading elements also provide radial stress grading,such as illustrated by the serially connected capacitors 82 and 84.

In the embodiment of the invention illustrated in FIG. 1, the stressgrading elements are illustrated as being straight cylinders, but theymay have other configurations, as required by a specific application.For example, FIG. 3 is a fragmentary elevational view, partially insection, of a bushing assembly which is similar to the bushing 10 shownin FIG. 1, except for the configuration of the stress grading elements.Like reference numerals in FIGS. 1 and 3 indicate like components.

More specifically, the stress grading elements of the bushing 100 shownin FIG. 3 are arranged into first and second groups 102 and 104, butinstead of using straight cylinders, the stress grading elements aresubstantially frustrum shaped, having a first diameter adjacent to oneend of the conductor 12, and a larger diameter facing the mounting means13 of the bushing. The first group 102 includes a plurality of stressgrading elements 106, 108, and 112, with the deviation of the elementsaway from a straight cylinder being illustrated by the angle 114relative to the stress grading element 112. The stress grading elementsof the first group 102 which are disposed in the encased end of thebushing 100 are also frustrum shaped, but oriented opposite to theorientation of the elements associated with the weather end of thebushing 100, i.e., the lower end of the element would have a smallerdiameter than the upper end. It should be noted that while the stressgrading elements of the first group 102 are in the shape of a frustrum,their wall portions are still substantially aligned, thus facilitatingmanufacture of the stress grading elements, such as by the successivecasting method.

The second group 104 includes a plurality of frustum shaped stressgrading elements 116, 118, and 122,

which are axially spaced with their wall portions aligned, and axiallydisplaced relative to the elements of the first group, such that theyoverlap the spaces between the elements of the first groups as describedrelative to the embodiment of the invention shown in FIG. 1.

While in most applications only two groups of axially spaced stressgrading elements will be required, in certain voltage ratings it may benecessary to utilize three or more tiers or groups of stress gradingelements, but in these instances the number of tiers will still besubstantially less than would be required with the conventional type ofcapacitor arrangement, and similar to the embodiment described using twogroups, there will be a maximum of two stress grading elements at anylocation perpendicular to conductor 12, regardless of the number ofgroups used. FIG. 4 is a fragmentary elevational view, partiallyinsection, of a bushing assembly 130 constructed according to anembodiment of the invention which has four tiers or groups of stressgrading elements, with like reference numerals in FIGS. 1 and 4indicating like components. Bushing assembly 130 includes first, second,third and fourth groups 132, 134, 136 and 138, respectively. The firstgroup 132 of stress grading elements, which group is closest toconductor 12, includes axially aligned elements 140 and 142 in theweather end of bushing 130, and a plurality of elements in the encasedend, such as two (not shown). The second group 134 includes elements oflarger diameter than the elements of the first group, and in includeselements 144, 146, 148 and 150 in the weather end, and a plurality inthe encased end (not shown). The first element 144 of the second group134 overlaps the first two elements of the first group 132. The thirdgroup 136 includes a plurality of stress grading elements 152, 154, 156and 158, in the weather end, and a plurality in the encased end, withthese elements being disposed in axially spaced relation such that theybridge the spaces between adjacent pairs of elements in the secondgroup, starting with second and third elements 146 and 148 of the secondgroup. The fourth group 138 includes a plurality of stress gradingelements such as elements 160 and 162, with these elements being axiallyspaced from one another, and axially displaced relative to the elementsof the third group, such that they bridge the spaces between adjacentpairs of elements in the third group, starting with third and fourthelements 156 and 158 of the third group. Thus, regardless of the numberof tiers, only two elements have overlapping ends at any locationperpendicular to conductor 12.

In summary, there has been disclosed a new and improved condenserbushing, which takes advantage of the excellent electrical strength ofcast solid insulation in puncture to provide a capacitive arrangementwhich facilitates the manufacture of cast type bushings. The number ofradial tiers required is reduced, and the number of axially spacedcapacitor elements is increased, to provide excellent longitudinalstress grading throughout the bushing, and also provide the requiredradial stress grading through the bushing structure. In cast typebushings, the number of axially spaced, aligned, stress grading elementspresents little problem to the manufacture thereof, resulting in anoverall lower manufacturing cost for the cast type bushing.

While the disclosed stress grading element structure is best suited foruse with cast solid insulation systems, it may also be used to advantagewith wound paper type construction, especially when the paper isimpregnated with resin. However, in certain applications it may bebeneficially used with the conventional paper-oil construction.

I claim as my invention:

1. An electrical bushing adapted for mounting in the casing ofelectrical apparatus, comprising:

an axially extending electrical conductor having first and second ends,

solid insulating means disposed about said electrical conductor, stressgrading means disposed in said solid insulating means, including atleast first and second groups of substantially tubular stress gradingelements disposed coaxially with said electrical conductor,

said first group of stress grading elements being axially spaced fromone another and dimensioned such that their wall portions aresubstantially in alignment,

said second group of stress grading elements being axially spaced fromone another and dimensioned such that their wall portions aresubstantially in alignment, with the resulting aligned group beingradially spaced from the first aligned group of stress grading elements,

said first and second groups of stress grading elements being axiallyoffset from one another such that each of the stress grading elements ofthe second group overlaps the ends of two stress grading elements, saidstress grading elements being disposed such that there is a maximum oftwo stress grading elements above ground potential at any locationperpendicular to said conductor.

2. The electrical bushing of claim 1 wherein at least certain of thestress grading elements are in the form of a frustrum.

3. The electrical bushing of claim 1 wherein at least certain of thestress grading elements are in the form of a straight cylinder.

4. The electrical bushing of claim 1 wherein at least certain of theelements are formed of a semiconductive material having a voltagedependent resistivity.

5. The electrical bushing of claim 1 wherein the axial lengths of thestress grading elements are substantially the same.

6. The electrical bushing of claim 1 wherein the axial lengths of atleast certain of the stress grading elements are different.

7. The electrical bushing of claim 1 wherein the axial spacing in eachgroup of stress grading elements is the same.

8. The electrical bushing of claim 1 wherein the axial spacing betweenthe stress grading elements in each group is changed in a predeterminedpattern.

9. The electrical bushing of claim 1 wherein the radial spacing betweenthe aligned groups is the same from one end of the bushing to the other.

10. The electrical bushing of claim 1 wherein the radial spacing betweenadjacent aligned groups changes in a predetermined relationship from oneend of the busing to the other.

11. The electrical bushing of claim 1 including at least one additionalgroup of stress grading elements, with the elements of the first andsecond groups, and the elements of the at least one additional groupbeing axially located such that each space between adjacent stressgrading elements of one group is bridged by a stress grading elementfrom an adjacent group, without any overlapping of elements of the firstand third groups.

12. An electrical bushing adapted for mounting in the casing ofelectrical apparatus, comprising:

an axially extending electrical conductor having first and second ends,

insulating means disposed about said electrical conductor,

stress grading means disposed in said insulating means, including aplurality of groups of substantially tubular stress grading elementsdisposed coaxially with said electrical conductor,

the elements in each of said groups being axially spaced from oneanother and dimensioned such that their wall portions are substantiallyin alignment,

each of said aligned groups being radially spaced from one another,

the elements in the plurality of groups being axially offset from oneanother such that each space between adjacent stress grading elements ofone group is bridged by a stress grading element from an adjacent group,with a maximum of two elements overlapping at any location perpendicularto said conductor.

1. An electrical bushing adapted for mounting in the casing ofelectrical apparatus, comprising: an axially extending electricalconductor having first and second ends, solid insulating means disposedabout said electrical conductor, stress grading means disposed in saidsolid insulating means, including at least first and second groups ofsubstantially tubular stress grading elements disposed coaxially withsaid electrical conductor, said first group of stress grading elementsbeing axially spaced from one another and dimensioned such that theirwall portions are substantially in alignment, said second group ofstress grading elements being axially spaced from one another anddimensioned such that their wall portions are substantially inalignment, with the resulting aligned group being radially spaced fromthe first aligned group of stress grading elements, said first andsecond groups of stress grading elements being axially offset from oneanother such that each of the stress grading elements of the secondgroup overlaps the ends of two stress grading elements, said stressgrading elements being disposed such that there is a maximum of twostress grading elements above ground potential at any locationperpendicular to said conductor.
 2. The electrical bushing of claim 1wherein at least certain of the stress grading elements are in the formof a frustrum.
 3. The electrical bushing of claim 1 wherein at leastcertain of the stress grading elements are in the form of a straightcylinder.
 4. The electrical bushing of claim 1 wherein at least certainof the elements are formed of a semiconductive material having a voltagedependent resistivity.
 5. The electrical bushing of claim 1 wherein theaxial lengths of the stress grading elements are substantially the same.6. The electrical bushing of claim 1 wherein the axial lengths of atleast certain of the stress grading elements are different.
 7. Theelectrical bushing of claim 1 wherein the axial spacing in each group ofstress grading elements is the same.
 8. The electrical bushing of claim1 wherein the axial spacing between the stress grading elements in eachgroup is changed in a predetermined pattern.
 9. The electrical bushingof claim 1 wherein the radial spacing between the aligned groups is thesame from one end of the bushing to the other.
 10. The electricalbushing of claim 1 wherein the radial spacing between adjacent alignedgroups changes in a predetermined relationship from one end of thebusing to the other.
 11. The electrical bushing of claim 1 including atleast one additional group of stress grading elements, with the elementsof the first and second groups, and the elements of the at least oneadditional group being axially located such that each space betweenadjacent stress grading elements of one group is bridged by a stressgrading element from an adjacent group, without any overlapping ofelements of the first and third groups.
 12. An electrical bushingadapted for mounting iN the casing of electrical apparatus, comprising:an axially extending electrical conductor having first and second ends,insulating means disposed about said electrical conductor, stressgrading means disposed in said insulating means, including a pluralityof groups of substantially tubular stress grading elements disposedcoaxially with said electrical conductor, the elements in each of saidgroups being axially spaced from one another and dimensioned such thattheir wall portions are substantially in alignment, each of said alignedgroups being radially spaced from one another, the elements in theplurality of groups being axially offset from one another such that eachspace between adjacent stress grading elements of one group is bridgedby a stress grading element from an adjacent group, with a maximum oftwo elements overlapping at any location perpendicular to saidconductor.